1. Field of the Invention
The present invention relates to a sheet conveying apparatus for conveying a sheet member such as a recording sheet and an image forming apparatus using such a sheet conveying apparatus.
2. Related Background Art
Image forming apparatuses having a pin-feed tractor have been used as output equipments for personal computers, word processors, facsimiles and the like.
In a conventional example shown in FIG. 14, a convey roller 50 and driven pinch rollers 51a, 51b for urging a sheet against the convey roller are used as sheet convey means. A so-called U-turn path is provided for conveying the sheet around the convey roller 50.
When a cut sheet is used as a recording medium, the cut sheet is pinched between the convey roller 50 and the pinch rollers 51a, 51b, thereby conveying the sheet by friction forces generated between these rollers. On the other hand, when a continuous sheet (also referred to as "fan-fold sheet" hereinafter) is used, pin holes formed in the fan-fold sheet are engaged by pins 52a of a pin-feed tractor 52, and the sheet is pinched between the convey roller 50 and the pinch rollers 51a, 51b, thereby conveying the sheet to a recording portion by friction forces generated between these rollers.
In any case, the recording is effected by a recording head 53 regarding the conveyed sheet, and thereafter, the sheet is shifted upwardly and discharged out of the apparatus by a discharge roller 54a and a pinch roller 54b for urging the sheet against the discharge roller. In the image forming apparatus having the U-turn path, an inlet opening and a discharge opening for the fan-fold sheet can be arranged at a rear portion of the apparatus, and the selection of and the switching between the fan-fold sheet and the cut sheet can be performed by manipulating a lever, in a condition that the sheets were set.
However, in order to convey several kinds of sheets (particularly, sheets having great resiliency such as post cards, envelopes and the like), it is necessary that a diameter of the convey roller 50 is increased to reduce load resistance. Further, in the image forming apparatus, when the sheet is intermittently conveyed by a predetermined amount, the smaller the diameter of the convey roller 50 the greater the rotational angle of the convey roller 50. To the contrary, although the greater the diameter of the convey roller 50 the smaller the rotational angle, it is hard to be influenced upon eccentricity of the convey roller 50.
Accordingly, the large diameter of the convey roller not only reduces the load resistance in the U-turn path, but also eliminates one of factors for worsening conveyance accuracy of the convey roller 50.
On the other hand, in view of serial-recording, the intermittent conveyance of sheet requires high accuracy; if the accuracy is poor, one-line recording area obtained by one scan of the recording head will be overlapped with the previous one-line recording area or be excessively spaced apart from the previous one-line recording area, thereby worsening the recording quality greatly. In many cases, in drive force transmission in a sheet conveying apparatus, a rotational force of a motor is transmitted to the convey roller through gears and/or belt; in order to provide predetermined torque of the convey roller, the drive force is transmitted with speed reduction. The most preferential factor for giving the influence upon the sheet conveyance accuracy is eccentricity of drive force transmitting members (such as gears, belt and the like) and eccentricity of the convey roller. In order to minimize the influence of such eccentricity, various efforts for improving the accuracy of such elements have been tried.
On the other hand, when the diameter of the convey roller 50 is increased, the speed reduction ratio of the rotational force of the motor and the number of speed reduction steps or stages are increased, the conveyance accuracy of the convey roller 50 is worsened.
As mentioned above, although it is desirable to improve the manufacturing accuracy of the drive force transmitting members and speed reduction members (such as gears) in order to enhance the conveyance accuracy, there are technical limitations and increase in cost. If the diameter of the convey roller 50 is decreased, certain kinds of sheets including the fan-fold sheet cannot be handled and the advantage of the U-turn path cannot be utilized.